Tubular metallic softball and baseball bats are well known in the art. A familiar example is a tubular aluminum bat. Particular embodiments of inserts for tubular baseball and softball bats are known in the art. Eggiman's U.S. Pat. No. 5,415,398 (Eggiman), which is incorporated herein by reference, concerns a softball bat, including an insert to provide structural support to the bat. The Background section of Eggiman discusses some prior designs produced in the continuing quest for bats with a better "slugging" capacity. Eggiman also discusses the desire to have bats with large elastic deflections, but without plastic deflection upon impact.
Eggiman discloses a tubular insert that fits within the frame of the bat, and is movable relative to the bat. In one embodiment the insert is separated from the frame by a gap that allows movement of the frame relative to the bat. Such an insert improves the impact response of the bat by increasing elastic deflection without allowing plastic deflection. The Eggiman softball bat is a considerable improvement over prior bat designs providing increased power transfer from the bat to the ball.
Nevertheless, the need for a bat with even better slugging capacity continues. It is desirable to improve the slugging capacity of bats without adding significant weight. This is particularly difficult in baseball bats where high impact forces require greater structural integrity within the bat to prevent unwanted plastic deformation. Prior baseball bats that provide this structural integrity typically added significant weight to the bat, did not allow sufficient elastic deformation within the bat, or decreased the diameter of the bat in the impact area to compensate for the added weight to the bat.
A further problem with prior bats is the size of the optimal hitting area or "sweet spot" of the bat. The sweet spot is typically located near the center of the impact area of the bat. The performance of the bat declines considerably when a ball impacts the bat outside the sweet spot, for example near the end of the bat. Typically, such an impact results in greater vibrations being felt by the user and decreased energy transfer from the bat to the ball.
Composite materials have been used to form the structural components of bats to improve their performance. An example is U.S. Pat. No. 5,364,095 by Easton. Easton discloses a tubular metal bat that is internally reinforced with a fiber composite sleeve. The Easton sleeve consists of multiple fiber layers, with each layer having bi-directional woven fibers that are directed at 0 and 90 degrees relative to the axis of the bat. The sleeve is formed so that it is bonded to the aluminum frame and it presses outwardly on the frame to produce a pre-load stress of several thousand pounds per square inch. This design does not allow for sufficient elastic deformation of the external frame of the bat. Moreover, the properties of the Easton composite sleeve are essentially isotropic.